SOLVENT CONTROL THE ELECTRONIC EXCITED STATE STRUCTURE: RAMAN
STUDY
B.K.Chowdhury1 and S.Umapathy1*
1
Indian Institute of Science; Bangalore 560012; India; E-Mail: [email protected]
Keywords: triplet excited state, time resolved resonance Raman spectra, solvent polarity
Abstract: Solvent plays an important role in the reactivity of chemical reaction dynamics. In this
paper, it is shown that the excited state structure of 2,2,2-trifluroacetophenone can be modified
using solvent polarity, with the use of time resolved Raman spectroscopy.
d if f . a b s o r b a n c e ( a . u )
2,2,2-trifluroacetophenone has two nearby triplet-excited states [1]. The current concept is that
in non-polar solvent the lowest triplet excited state is π-π* in nature with a close lying n-π* state
[2]. We have carried out flash photolysis and time Resolved Resonance Raman study in four
solvents of differing polarity, namely, CCl4 as non-polar solvent, acetonitrile and propionitrile as
polar solvents and CHCl3 as hydrogen bonding solvent.
The flash photolysis spectra in the non-polar CCL4 and the polar acetonitrile are given in Fig1.
We have used 266 nm as pump and 355 nm as probe wavelengths for time resolved resonance
Raman study. The Time Resolved Resonance Raman spectra in CCl4 and Acetonitrile have been
shown in Fig 2. We find that in non-polar solvent the lowest triplet state is n-π* in nature and in
polar it is π-π*. It is known that carbonyl n-π* excited state has higher hydrogen abstraction ability
than the π-π* excited state. One of the products of this photochemical reaction, ketyl radical, in the
presence of CF3 group forms a cyclic 5 membered ring through hydrogen bonding and as a result
the back hydrogen transfer rate becomes faster. This leads to reduction in the ketyl radical quantum
yield. We have used ab-initio and DFT methods to determine the structure and vibrational
frequency of the lowest triplet state in presence of the chosen solvents.
In A c e to n itr ile
In C C l4
3 4 0
3 6 0
3 8 0
4 0 0
4 2 0
4 4 0
4 6 0
4 8 0
W a v e le n g t h ( n m )
Fig 1 Transient absorption spectra with 50 ns delay.
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5 0 0
Intensity (a.u)
1372
1420
1141
(a)
1516
1553
1296
1000 ns
400 ns
200 ns
50 ns
1000
1200
1400
1600
1800
2000
Intensity (a.u)
Probe only
(b)
1000 ns
400 ns
200 ns
1374 1428
1173
50 ns
- 500 ns
1000
1200
1400
1600
1800
-1
Raman Shift (cm )
Fig2 Time Resolved Resonance Raman spectra in (a) Acetonitrile and (b) CCl4
Acknowledgements:
We would like to thank Indian Institute of Science, Department of Science and Technology (DST)
and Council of Scientific and Industrial research (CSIR) for financial support.
References:
1. Wagner, P.J.Leavitt, R.A; J.Am.Chem.Soc. 1973,95,3669
2. Wagner, P.J; Truman, R.J; Scaiano, J.C. J.Am.Chem.Soc. 1985,107,7093-7097
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